I found a couple of articles on the PTU/RDU for ya. Z Since the Power Transfer Unit operates all the time to spin the driveshaft to the rear, the PTU can get hot. On the police version, a number of steps are taken to assure reliability and durability in police use. First, a cooler is added to the front of the PTU housing. The police-only PTU housing has an integral water jacket connected with water lines that run to the bottom three coils of the radiator. The jacket receives engine coolant from the radiator to maintain the correct PTU oil temperature.
Second, the PTU has a gear lube temperature sensor. A module keeps track of the gear lube temp, and how long / how often it was at that temperature. When a threshold is reached, a change PTU Oil light activated.
Finally, the AWD system has a two-stage way to protect itself from damage during extreme, prolonged use. The ATC as a heat protection strategy based on a number of factors, including wheel speed and duration of activity, to determine if overheating is occurring. If the ATC detects overheating, it will lock the clutches together to cool them. Clutches that slip to provide partial torque transfer get hot from the friction of the slippage. Locking the clutches stops the slip and cools the entire unit. In a locked condition, power is transferred to the rear wheels.
If the temp continues to rise even in a locked condition, the PCM disables the coupling altogether. The vehicle operates only as a FWD. AWD OFF is displayed. On the Police Interceptors, the PCM will automatically exit heat protection mode and clear the AWD OFF message when the temp returns to normal. This whole process is sped up by turning the ignition off and allowing the vehicle to cool for 10 minutes.
The ATC/RDU used on the Police Interceptors uses fluid-filled "wet" clutch packs. The retail Taurus and Explorer use dry clutches. The PI wet clutches can withstand higher temps and withstand higher temps longer than the retail unit. The ATC and RDU are not field serviceable. In case of a problem with either system, the whole unit is replaced.Police-only Cooling
The police version of AWD has three unique features to improve durability during severe use. First, the Power Transfer Unit on the AWD system uses a police-only auxiliary cooler. The retail AWD unit is air cooled, i.e., the aluminum housing dissipates heat to the surrounding air. When the vehicle is stationary, the amount of cooling is obviously less than when the vehicle is moving.
On the police AWD unit, the front housing of the Power Transfer Unit has a special water jacket cast into the assembly. The integral PTU auxiliary cooler is part of the water cooling system. That means the cooling remains "active" since the cooling water is constantly circulated, whether the police vehicle is moving or stationary. The PTU cooler is plumbed into the bottom of the radiator, where the water is cooler than at the top of the radiator.
Second, the Power Transfer Unit uses an internal temperature sensor, not found on the retail unit. A module keeps track of oil temperature in the PTU and the amount of time spent at that temperature. An electric clutch pack engages and disengages as torque is transferred to the rear wheels.
Both the PI Sedan and PI Utility default to FWD under normal conditions. As the front wheels slip, torque is selectively diverted to the rear wheels. A time versus temp logarithm activates an Oil Minder light in the driver message center if and when it is time to change the PTU fluid. The special synthetic oil may last 100K miles in normal police use.
Third, the Rear Differential Unit on the police version uses a fluid-filled clutch pack, while the retail version uses dry clutches. The fluid-filled RDU can withstand more severe duty.
The cooling ability of the police-only auxiliary radiator-water jacket combination was tested in 110 deg. F weather. Hard driving—where power is constantly shifting from front to rear, back and forth—can take many different forms.
Think aggressive driving during a city pursuit: hard throttle for a city block, 90-degree turn, hard throttle, where torque definitely shifts from front to rear. Also think getting unstuck in desert sand. Even though the vehicle and wheel speeds are not high, torque is constantly shifting between the front and rear spinning wheels.
Under the worst conditions of high-torque, clutch engagement and disengagement, the synthetic oil reached temperatures around 195 deg F. The special lube is rated for temps just over 500 deg F. The air-to-water cooler works... Some AWD drivetrains are so sensitive that you must have exactly the same tread depth/tire wear on all four times. If your times are half worn and you must replace one tire with a new one, you must actually replace all four tires so the trend depth (overall tire height-diameter) stays the same.
Not so with the Ford AWD system. The AWD controllers are tolerant of any tread depth differences. The PI Sedan and PI Utility can accommodate two new tires on the front and two half-worn tries on the rear, or one new one or three new one.....Hope this helps any. Z
